Preservative-treated wood products are well known to significantly prolong service life, and thereby extend the forest resource and enhance its sustainability. Inevitably, however, the treated products become unserviceable either due to mechanical damage or failure, biological deterioration, or obsolescence. It is estimated that about 5 million tons of spent preserved wood is disposed of annually into landfills in the Untied States, and the ratio of wood treated with chromated copper arsenate (CCA) is forecasted to increase significantly due to its use in housing and decking. These CCA-treated posts and sleepers have an average working life of approximately 25 years, therefore the release of CCA-treated wood products is expected to increase continuously over the next decades.
Disposal of the spent CCA-treated wood has become a major concern because of its residual toxic CCA content, in particular the arsenic and chrome. Conventional waste disposal options for spent preserved wood, such as burning and landfilling, are becoming more and more costly or even impractical because of increasingly strict regulatory requirements. The burning of treated wood may be extremely dangerous and even more so when the wood has been treated with CCA and this not only in respect to the possible environmental pollution but also where public health is concerned. Studies have shown that burning of the preservative-treated wood waste emits highly toxic smoke and fumes in the environment. In the case of landfills, it is necessary to remove the preservatives in order to meet the landfill regulations. For example, studies have shown that CCA compounds can be gradually leached out in aquatic environments, for example by rain water. Moreover, it also causes the concern of space requirements.
Spent preservative-treated wood products are a potential recycling resource for the production of energy. Additionally, materials having additional value may be produced from these recycled materials. Wood-preservatives may also be recovered in the recycle operation. Thus, both removal of the toxic preservatives from the preservative-treated wood and recycling of the detoxified preserved wood are of great importance to those concerned with the life cycle management of treated wood.
The recycling of CCA-treated wood products is presently undertaken to a very limited degree due to the difficulty in complying with environmental protection requirements and regulations as well as for economic reasons.
One of the recycling options for preserved wood is composite manufacturing. However, preservative interference with fiber/adhesive bonding and the volatile properties of the preservative components at the high temperature can cause processing and industrial hygiene problems. Further research and development are needed before this recycling option is adapted in commercial production. Moreover, the presence of toxic preservatives in the composite greatly limits the application area of the composite and this also produces obstacles to the commercial application of this recycling option.
Numerous studies and experiments have been carried out on combustion of the preservative-treated wood products. It was found that burning the preserved-wood, particularly that treated with CCA, in traditional incinerators is not possible because it emits highly toxic smoke and fumes. Most of arsenic contained in the CCA-treated wood evaporates and takes with it other heavy metals because of the thermal shock between ambient air and incinerator temperature. Above 450° C., arsenic trioxide particulate matter, produced in the incineration process, becomes extremely fine. It is difficult to trap except by using very expensive methods to capture and wash the arsenic trioxide containing smoke. Recently, a new combustion system has been prepared which is a vertically disposed reactor column, containing two sections. The lower sector of the reactor column is for heating the wood chips to about 400° C. (below the ignition point of the wood chips) by introducing hot gases having a low oxygen content. The upper section of the reactor column is continuously filled with humidified wood chips to progressively cool the gases to a temperature below the condensation temperature of the gases (about 65° C.) and deposit condensed particulates in the form of heavy metals on the wood chips. This process enable the control and blocking of arsenic vapor arising from the combustion of wood treated with CCA. However, this process is an energy-intensive one, having a low processing speed. Its economic effectiveness is dubious.
Conventional pyrolysis systems (fixed bed, batch or grate; fluidized bed; rotary kiln, etc.) operate at too high temperature to prevent the emission of metal vapors. Percentages of arsenic volatilized have been reported to range between 8 and 95%, even though, the amounts of copper and chromium volatilized are found to be much lower than that of arsenic. A low-temperature (300–450° C.) pyrolysis system for the CCA-treated wood is reported to be effective in decreasing the release of arsenic during the pyrolysis of CCA-treated wood. This improved pyrolysis process has not proven to be economically viable.
Based on the consideration of environmental issues and associated restrictions, recently many attempts have been made to remove CCA from the CCA-treated wood by using solvent extraction, biological remediation and the like.
The solvent extraction of preservatives from spent treated wood is to solvate or dissolve the preservatives, removing them from the wood to the level compatible with the succeeding treatment or utilization processes, such as combustion, composite manufacturing, biodegradation processes and the like. The extraction process may be effective for high CCA concentrations. However, among the disadvantages of this recycling method are the huge amount of chemical solvents used, the long duration of the process, and the problems associated with handling and recycling of resulting extraction solution containing the toxic CCA components. Recently it is reported in a Japanese patent that over 90% of the copper, chromium and arsenic can be extracted out from the spent CCA-treated wood by supper critical carbon dioxide. Compared with the common solvent extraction, this technique has a shorter duration and high removal ratios of copper, chromium and arsenic but requires less solvent. However, it is still in a lab-scale and its economic effectiveness and possible treatment scale are not clear.
With a biological approach, there are some encouraging results in using both fungi and bacteria to release copper, chromium and arsenic from the treated wood. However microbiological separation is still at an early stage and offers no industrial application to solve the problem at hand. The main disadvantages of biological degradation are the inability of many organisms to tolerate or metabolize wood that contains high preservative concentrations. Therefore, a preliminary preservatives extraction process is needed for treated wood with a high concentration of preservatives. Moreover, the biological approach takes a long time to digest preservatives, and the economics of the whole process require much more attention.
The above review of the prior art indicates that up to now almost all the effort of recycling CCA-treated wood have been focused on treatments for environmentally safe disposal. However, recycling of the treated wood and the recovery of chromated copper arsenate has been largely ignored. As mentioned above, huge amount of preserved wood are removed from service annually. It would be very important if there were an economically effective process wherein both CCA and wood can be recovered respectively for recycling. Based on this consideration, recently, we have developed a novel process for the recycling of the spent CCA-treated wood. By this process, chromated copper arsenate is recovered for recycling and the detoxified wood can be used as bio-based chemicals for preparation of resin or plastics.
The process of the present invention comprises four steps: liquefaction of CCA-treated wood; separation of chromated copper arsenate from the liquefied wood solution; utilization of detoxified liquefied wood to polymeric materials; and regeneration of chromated copper arsenate.
In the process, CCA-treated wood is first liquefied in the presence of aliphatic or aromatic hydroxyl-containing substances, such as polyhydroxy alcohols or phenols, and some CCA recovering-enhancing additives, such as ferrous salts at a temperature of 100–170° C. with acidic catalyst or at elevated temperatures of 200–250° C. without catalyst. Aliphatic hydroxyl-containing substances also include polyether polyols, polyester polyols and mixtures thereof as well as the above-noted polyhydroxy substances in the step (a) liquefaction of CCA treated wood. An illustrative aliphatic hydroxy-containing substance is a mixture of glycerol and polyethylene glycol having a molecular weight from about 200 to about 1500. The amount of glycerol in the mixture of glycerol and polyethylene glycol is from about 2% to about 50% by weight. Additionally, the aliphatic hydroxy-containing substance may be a mixture of ethylene glycol and polyethylene glycol having a molecular weight from about 200 to about 1500. The amount of glycerol in the mixture of ethylene glycol and polyethylene glycol is from about 2% to about 50% by weight. A cyclic ester my also be added to the aliphatic hydroxyl-containing substance in such liquefaction step (a). This treatment converts the spent wood into a thick liquid with molecular weights ranging from several hundreds to several thousands. As a second step, the liquefied wood is diluted with water or aqueous organic solvent to form a solution for easier processing. To this solution, precipitants or complexing agents for the hazardous elements chromium, copper and arsenic, (sometimes referred to herein as “heavy metals”) such as calcium hydroxide or phosphoric acid (in the ease where it was not used in the liquefaction) are added and the solution is agitated and then allowed to sedimentation. The sediment, which contains the heavy metals copper, chromium and arsenic, is then separated from the solution by centrifugation or filtration. By this process, more than 95% of the copper, chromium and arsenic are removed from the liquefied CCA-treated wood solution. A detoxified liquefied CCA-treated wood solution is therefore produced. As a third step, the detoxified liquefied CCA-treated wood solution is then concentrated to remove the diluting solvent and the resulting concentrated detoxified liquefied CCA-treated wood solution is used as a bio-based raw matenals for the preparation of polymer materials, such as polyurethanes and phenoiic resins. The diluting solvent recovered can be reused in the process without further treatment. Finally, the sediment containing chromium, copper and arsenic is treated with concentrated inorganic acid such as sulfuric, nitric or phosphoric acid to regenerate the chromated copper arsenate, which may be reused in the preservation industries of wood.
Since the process uses relative low temperature, short reaction time, and relatively small amount of organic reagents, it is an effective and economically feasible technique for recycling of spent CCA-treated wood.